The art of lithographic printing is based upon the immiscibility of oil and water, wherein the oily material or ink is preferentially retained by the image area and the water or fountain solution is preferentially retained by the non-image area. When a suitably prepared surface is moistened with water and an ink is then applied, the background or non-image area retains the water and repels the ink while the image area accepts the ink and repels the water. The ink on the image area is then transferred to the surface of a material upon which the image is to be reproduced; such as paper, cloth and the like. Commonly the ink is transferred to an intermediate material called the blanket which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
A very widely used type of lithographic printing plate has a light-sensitive coating applied to an aluminum base support. The coating may respond to light by having the portion which is exposed become soluble so that it is removed in the developing process. Such a plate is referred to as positive-working. Conversely, when that portion of the coating which is exposed becomes hardened, the plate is referred to as negative-working. In both instances the image area remaining is ink-receptive or oleophilic and the non-image area or background is water-receptive or hydrophilic. The differentiation between image and non-image areas is made in the exposure process where a film is applied to the plate with a vacuum to insure good contact. The plate is then exposed to a light source, a portion of which is composed of UV radiation. In the instance where a positive plate is used, the area on the film that corresponds to the image on the plate is opaque so that no light will strike the plate, whereas the area on the film that corresponds to the non-image area is clear and permits the transmission of light to the coating which then becomes more soluble and is removed. In the case of a negative plate the converse is true. The area on the film corresponding to the image area is clear while the non-image area is opaque. The coating under the clear area of film is hardened by the action of light while the area not struck by light is removed. The light-hardened surface of a negative plate is therefore oleophilic and will accept ink while the non-image area which has had the coating removed through the action of a developer is desensitized and is therefore hydrophilic.
The manufacture of a printing plate which can be employed as either a positive-working plate or a negative-working plate has been proposed heretofore. Such a plate has the advantage of increased versatility since either positive or negative originals can be used.
One example of a printing plate which can be employed as either a positive-working plate or a negative-working plate is that described in British Patent No. 2,082,339, published Mar. 3, 1982 and in U.S. Pat. No. 4,927,741, issued May 22, 1990. These patents describe a lithographic printing plate in which the imaging layer comprises a resole resin and an orthoquinone diazide and, optionally, also contains a novolac resin. Such a plate can be used as a positive-working plate by a process comprising the steps of imagewise exposure and development with an aqueous alkaline developing solution to remove the coating from the exposed areas. Alternatively, it can be used as a negative-working plate by a process comprising the steps of imagewise exposure, heating of the plate to convert the coating in the exposed areas to an insoluble form, overall exposure of the plate to radiation to render previously unexposed areas soluble, and development with an aqueous alkaline developing solution to remove such soluble areas.
A further example of a printing plate which can be employed as either a positive-working plate or a negative-working plate is that described in U.S. Pat. No. 4,708,925, issued Nov. 24, 1987. This patent describes a lithographic printing plate in which the imaging layer comprises a phenolic resin and a radiation-sensitive onium salt. As described in the patent, the interaction of the phenolic resin and the onium salt produces an alkali-insoluble composition which is restored to alkali-solubility upon photolytic decomposition of the onium salt. The printing plate can be utilized as a positive-working plate or as a negative-working plate using the same process steps as are described hereinabove in regard to the plates of British Patent 2,082,339 and U.S. Pat. No. 4,927,741.
The radiation-sensitive composition described in U.S. Pat. No. 4,708,925 is useful for the preparation of a direct laser addressable plate. Digital imaging information can be used to image the plate without the need to utilize an imaging master such as a photographic transparency.
The hereinabove described printing plates of the prior art which can be employed as either a positive-working plate or a negative-working plate are lacking in one or more desirable features. Thus, the plates described in British Patent No. 2,082,339 are not infrared-sensitive and thus cannot be digitally imaged by a laser which emits in the infrared region. Moreover, both the plates of British Patent No. 2,082,339 and U.S. Pat. No. 4,708,925 require two exposure steps in order to be utilized as a negative-working plate, i.e., an imagewise exposure and a subsequent overall exposure. This adds greatly to the cost and complexity of the process. The difficulties of carrying out a process requiring multiple exposure steps, while meeting the requirements of the trade for high speed processing, are described in U.S. Pat. No. 4,927,741.
It is toward the objective of providing an improved printing plate, which is capable of use as either a positive-working plate or a negative-working plate, which is useful as a direct laser addressable plate and which can be processed by a simple procedure that requires only a single exposure, that the present invention is directed.